Journal of Nanomaterials & Molecular NanotechnologyISSN: 2324-8777

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Research Article, J Nanomater Mol Nanotechnol Vol: 7 Issue: 3

Characterization of Anatase and Rutile Phase of TiO2 Nanostructures with Different Thermal Annealing

Napon Butrach, Orathai Thumthan and Suttinart Noothongkaew*

Department of Physics, Faculty of Science, Ubon Ratchathani University, 34190 Thailand

*Corresponding Author : Suttinart Noothongkaew
Department of Physics, Faculty of Science, Ubon Ratchathani University, 34190 Thailand
E-mail: [email protected]

Received: March 14, 2018 Accepted: June 01, 2018 Published: June 07, 2018

Citation: Butrach N, Thumthan O, Noothongkaew S (2018) Characterization of Anatase and Rutile Phase of TiO2 Nanostructures with Different Thermal Annealing. J Nanomater Mol Nanotechnol 7:3. doi: 10.4172/2324-8777.1000247

Abstract

TiO2 nanostructures were prepared by anodization of Ti foils. The TiO2 nanostructure films were annealed at the temperature range of 500°C to 900°C for 2 h. The morphology, elemental composition, and crystallization of TiO2 nanostructures were analyzed by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Raman spectra, and X-ray spectroscopy (XPS), respectively. XRD and Raman spectra results confirm the presence of the anatase phase for TiO2 nanostructure films which were annealed at 500°C to 700°C. Furthermore, it found that anatase to rutile phase transition occurred at temperature above 700ºC.

Keywords: Titanium dioxide; Nanotubes; Thermal annealing; Anodization

Introduction

In recent years, TiO2 nanostructures have been greatly interested because of their specific physical and chemical properties such as excellent catalytic activity, dielectric, electronic, gas sensing properties and so on. Due to these properties, many applications of TiO2 nanostructures have been reported such as gas sensors, hydrogen generation by water photoelectrolysis, dye sensitized solar cells, photocatalysts, and so forth [1-12]. However, many applications based on TiO2 require high purity TiO2 with controlled crystallite size, definite phase composition, and surface properties. Therefore, it is important to control the morphology and crystalline phase of the TiO2 system. It is well known that bulk TiO2 exists in three crystalline types [13-15]; anatase, rutile, and brookite phases. Among these phases, TiO2 exists mostly in anatase and rutile phases. These two phases exhibit different properties and photocatalytic performance. Since the anatase phase is chemically and optically active, it is suitable for photocatalytic, while the rutile phase has high refractive index and has good stability at high temperature. However, anatase phase can be transformed into rutile phase when the samples are calcined at higher temperature. This work aims to synthesis high quality of TiO2 nanostructures fabricated by anodization of Ti foils [16-22], and study the transformation of anatase to rutile phase after thermal annealing at 500ºC to 900ºC.

Experimental Details

TiO2 nanostructures were fabricated on Ti foil substrates by using electrochemical anodization technique. The experimental details have been previously reported by our works [23]. Briefly, clean Ti foil was anodized at room temperature in a two-electrode electrochemical cell in which the graphite sheet and Ti foil were used as the cathode and anode, respectively. Ti foil was anodized in mixture of NH4F, ethylene glycol, and 12 wt % of deionized water for 2 h under a constant voltage of 50 V. After anodization, the sample was cleaned with DI water, acetone, and ethanol, respectively, for 10 min in each process. Then, the resulting oxide layers were annealed at several temperatures in the range of 500°C to 900°C for 2 h. The morphological and crystal phase identification of TiO2 nanostructures were characterized by Field emission scanning electron microscope (FE-SEM), X-ray diffraction analysis (XRD), and Raman technique, respectively. The chemical state and surface composition of samples were determined by X-ray photoelectron spectroscopy (XPS).

Results and Discussion

The morphology of TiO2 nanostructures after annealing at 500°C to 900°C were characterized by FE-SEM. Figure 1(a) shows top view of the anodized TiO2 nanostructures after annealing at temperature of 500°C. The surface of sample shows smooth and high density of tubes. The average inside diameter of the tubes was approximately 200 nm after thermal annealing at 700°C, 800°C, and 900°C, as shown in Figures 1(b-d), respectively. We found that the surface of tubes was damaged after increasing the thermal annealing from 700°C to 900°C, and TiO2 anatase phase was transformed to rutile phase. This result can be explained that anatase phase of TiO2 tends to transform into rutile phase. In addition, the surface area decreases and, thus, induces a loss of catalyticity. Crystal structure of sample can be confirmed by XRD spectra as shown in Figure 2.

Figure 1: Shows FE-SEM images of TiO2 nanostructures at annealing temperature of (a) 500°C, (b) 700°C, (c) 800°C, and (d) 900°C, respectively.

Figure 2: Shows XRD patterns of TiO2 nanostructures before and after thermal annealing at temperature of 500°C, 700°C, 800°C, and 900°C, respectively.

The XRD technique was used to characterize TiO2 nanostructures after thermal annealing at temperature of 500°C, 700°C, 800°C and 900°C, respectively. The crystalline phase of sample can be identified from XRD patterns as shown in Figure 2. The diffraction peaks at 2θ are 25.2°, 37.8°, 47.8°, 52.9°, and 62.9° which correspond to (101), (004), (200), (105), and (204) of the anatase phase of TiO2 (JCPDS, card No. 21-1272) [20,24,25]. All the major peaks are TiO2. The results indicate that TiO2 nanostructures, after thermal annealing at 500ºC to 700ºC, are dominantly oriented in these plane directions. The featured peaks at 2θ of 40.3° and 70.8° are indexed to crystal planes of metal Ti phase. While peaks at 2θ of 27.35°, 55.1°, and 68.6°are indexed to (110), (220), and (301) rutile phase of TiO2. The diffraction patterns of the films annealed at 800°C to 900°C contain peaks of both anatase and rutile phase that indicate the occurrence of transformation from anatase phase to rutile phase between 700°C and 900°C, with the increase in annealing temperatures. It was observed that complete conversion of anatase to rutile cannot occur at 500°C in anodization of Ti foil. The various impurity peaks are not observed in the XRD spectrum, indicating that pure TiO2 samples obtained.

The Raman spectrum was used to confirm the formation of TiO2 anatase and rutile phase after thermal annealing at 500°C to 900°C as shown in Figures 3(a-d), respectively. According to Figures 3(a) and (b), it found that Ti foils annealed at 500ºC and 700ºC clearly show Raman peaks corresponding to the anatase phase of TiO2. The peaks at ~147, ~395.7, ~516.6, and ~637.5 cm-1 could be assigned to the Eg, B1g, B1g+A1g, and Eg, LO-phonon modes of the anatase structure, respectively. While, the Raman spectra of samples annealed at 800°C and 900°C are shown in Figures 3(c) and (d). Two bands at 445.3, and 625 cm-1 are in agreement with data observed in the spectra of a rutile phase [26-28]. The formation of anatase and rutile phases after thermal annealing of 500°C to 900°C are also confirmed from the XRD results (Figure 2).

Figure 3: Shows Raman spectra of TiO2 nanostructures before and after thermal annealing at (a) 500°C, (b) 700°C, (c) 800°C, and (d) 900°C, respectively.

XPS technique was used to determine the surface composition and chemical states of TiO2 nanostructures. Figure 4(a) shows core level XPS spectrum of O1s pre- and post-thermal annealing at 500°C to 900°C. Figure 4(a) shows the main peak at binding energy of 529.5 eV. The peak was shifted to higher binding energy with increasing the annealing temperature due to the metallic oxide Ti-O bonds which are consistent with binding energy of O2- in the TiO2 lattice. In addition, the peak appearing at 531.8 eV is assigned to the adsorbed OH- on the surface of TiO2 [18-20]. In core level XPS spectrum of TiO2 p as shown in Figure 4(b), two peaks at binding energy of 457.4 eV and 463.2 eV were also shifted to higher binding energy after increasing annealing temperature. These two peaks of TiO2 could be assigned to the levels of Ti4+2p3/2 and Ti4+2p1/2, respectively.

Figure 4: Shows high resolution XPS spectra of TiO2 before and after thermal annealing at 500°C to 900°C, (a) O1s, and (b) Ti2p.

Conclusion

TiO2 nanostructures were prepared by anodization method from Ti foil. And, the TiO2 films were annealed at temperature range of 500°C to 900°C for 2 h. We demonstrated that annealing of the TiO2 film leads to crystallization in the anatase phase at 500°C. The diffraction patterns of the films annealed at 800°C to 900°C contain peaks of both anatase and rutile reflections. Conversion of anatase to rutile phase occurred at the temperature interval of 800°C to 900°C. High surface area TiO2 is commonly formed by the anatase phase. However, it tends to transform into rutile phase at higher temperature. Therefore, the surface area decreases and induces a loss of catalytic activity.

Acknowledgements

Authors would like to thank Korea Research Institute of Chemical Technology, (KRIC), South Korea for experimental support and Ubon Ratchathani University for financial support to this research.

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